There are various methods for tightening thread joints: tightening to a predetermined torque, tightening to a predetermined angle, tightening to the elastic limit, tightening in the range of plastic deformations and others.
A method for tightening thread joints is known (RU Patent 2025270, published Dec. 30, 1994) comprising the measuring of the bolting torque transmitted on the thread element, its unbolting torque, and the calculation of the amount of tightening force, characterised in that the tightening force is calculated and the amount of bolting and unbolting torque is measured. The amount of calculated tightening force is compared to a predetermined amount, the amount of its deviance from the predetermined amount is determined, based on it the required deflection angle of the thread element is calculated to provide the predetermined amount of tightening force, the thread element is rotated to that angle, and when there is no deviation of the amount of force from the predetermined one, a preliminary bolting torque is transmitted to it.
A shortcoming is the high complexity and amount of work required for carrying out the method, since the bolting torque and the unbolting torque have to be measured and the deflection angle of the thread element has to be calculated. The method can be employed in individual cases due to the technical specification of its implementation.
A method for tightening detachable thread joints is known (RU Patent 2105620, published Feb. 27, 1998) comprising the insertion of an intermediate tightening part between the parts to be connected, which can simultaneously function as a seal of the joint. The tightening part is cooled to a temperature significantly below that of the parts to be connected before it is positioned, subsequently it is positioned between the parts to be connected, and then it is preliminarily tightened, for instance with a thread joint. The final tightening of the joint is carried out by forces of thermal expansion corresponding to the increase of the temperature of the intermediate tightening part under the influence of the environment as its linear dimensions increase.
The increase of the linear dimensions of the tightening part is calculated depending on the required tightening force. The greater the required force, the more the size of the intermediate part must decrease during cooling. This can be achieved by several methods: decreasing the cooling temperature; using a material with a high linear expansion coefficient; increasing the size of the part, which has an influence on the tightening force.
A shortcoming of the method is the high complexity and the amount of work required for its implementation due to the necessity of cooling equipment and the insertion of an additional part into the thread joint. The method has a rather small range of applications due to the technical specification of its implementation.
A method for tightening thread joints is known (USSR Certificate of Authorship 795928, published Jan. 15, 1981) wherein to increase the accuracy of a tightening torque of the thread joint a preliminary rotation of one thread part in the elastic deformation zone is carried out, the rotating angle and the torque corresponding to that position as well as the torque at a subsequent loosening of the thread joint are measured, and the rotating angle of the thread part is determined, providing the required tightening torque as a relation of the product of the preliminary tightening angle, the thread pitch and the required tightening force to the product of the number K and the difference of the preliminarily applied torque and the torque during the subsequent loosening.
A shortcoming of the method is the high complexity and the amount of work required for its implementation due to the necessity to rotate the thread parts, to measure the angles and torques and do the calculations. The method can be applied in individual cases due to the technical specification of its implementation.
Also known is a number of pertinent inventions analyzed by the applicant: U.S. Pat. No. 4,790,703, JP 2010096210, DE 19503962, CN 202768602, KR 101052583.
The most pertinent technical solution is a method applied on a differential thread joint (see P. I. Orlov, Osnovy konstruirovaniya. Spravochno-metodicheskoye posobie. T2. “Mashinostroyenie”, Moscow 1988). In this method a connector, namely a bolt with a differential thread, is used to join a base part and a part to be attached. On the bolt shaft two thread belts having a thread with the same direction are provided. The thread pitch of one belt is slightly larger than the thread pitch of the other belt. With the first thread belt the bolt is screwed into one of the parts to be tightened, and with the second it is simultaneously screwed into the other part. At one turn of the bolt the parts to be tightened approach each other to the amount of the difference of the pitches S2−S1. The bolt with the differential thread is equivalent to a regular threaded bolt with a very small pitch ΔS=S2−S1. The tightening can be carried out with a large force at a limited torque.
The shortcomings of this technical solution are the limited technical possibilities due to the simultaneous moving of the parts to be joined to a small amount, the poor coupling accuracy since no contact can be made in the predetermined place due to the constructive features of the parts, and also the poor reliability, since no strong tightening is possible due to the indeterminacy of the joining place of the parts.